1. Field of the Invention
The present invention relates to semiconductor processes, and more particularly, to a method for fabricating a wire bonding structure.
2. Description of Related Art
Current semiconductor packaging technologies generally involve performing wire bonding processes to form gold or copper bonding wires for electrically connecting semiconductor chips and carriers such as lead frames or packaging substrates. Further, the carriers are electrically connected to external devices such as circuit boards through conductive elements, for example, solder balls.
To perform such a wire bonding process, a ball end of a bonding wire is formed by the tip of a capillary through ultrasonic vibration on a bonding pad of a semiconductor device or a carrier and then bonded to the bonding pad by pressure. Thereafter, a wire body of the bonding wire is extended from the ball end and bonded by pressure to a bonding pad of another semiconductor device or carrier. Thereafter, the bonding wire is cut by the tip of the capillary.
FIG. 1A is a schematic cross-sectional view of a conventional wire bonding structure 1. Referring to FIG. 1A, a ball end 12a of a bonding wire 12 is formed by an ultrasonic vibration device through a scrubbing process on a bonding pad 11 of a substrate 10. Then, the ball end 12a is bonded to the bonding pad 11 by pressure. Thereafter, the wire body of the bonding wire 12 is extended from the ball end 12a. 
During the scrubbing process for forming the ball end 12a, particles and organic thin films generated during fabrication of the substrate 10 can be removed. Further, the scrubbing process is performed by the ultrasonic vibration device along a path in a vertical direction (Y direction) or a horizontal direction (X direction) to form a solder material on the bonding pad 11, thereby forming the ball end 12a, as shown in FIG. 1B.
The above-described scrubbing process is suitable for forming a ball end on the bonding pad 11 of large size, for example, on the bonding pad 11 having a width d greater than 40 um.
However, as circuits are developed toward fine pitches, the width d of the bonding pad 11 is continuously reduced. Accordingly, the size of the ball end 12a needs to be reduced. When the width d of the bonding pad 11 is reduced below 40 um, if the ball end 12a is formed through the above-described scrubbing process, delamination easily occurs between the ball end 12a and the bonding pad 11.
Further, the bonding wire 12 is generally made of a hard material such as copper or gold. When such a bonding wire is bonded to a very thin substrate such as a semiconductor chip, the bonding pad is easily damaged by the above-described scrubbing process for forming the ball end 12a. 
Therefore, there is a need to provide a method for fabricating a wire bonding structure so as to overcome the above-described drawbacks.